1. Field of the Invention
The present invention relates to a piezoelectric device, and more particularly to an energy trapping type piezoelectric device operable to use, e.g., a thickness extensional third harmonic vibration.
2. Description of the Related Art
Conventionally, energy trapping piezoelectric devices have been proposed, each of which has one pair of energy trapping vibration electrodes formed on opposite of the surfaces of a piezoelectric substrate to excite a thickness extensional third harmonic vibration. For a given material, the thickness extensional third harmonic vibration has a frequency constant which is about three times larger than that of the fundamental wave.
For piezoelectric devices which use a fundamental wave, the upper limit of the center frequency is about 15 MHz, considering the restrictions in strength of the piezoelectric substrates. On the other hand, the upper limits in center frequency of piezoelectric device using third harmonic waves are about 30 to 40 MHz. That is, the piezoelectric device can be used in a higher frequency range.
The optimum electrode structures of energy trapping piezoelectric devices operable to excite a thickness extensional third harmonic vibration are different, depending on type of material used. Thus, it is necessary to determine the optimum electrode size for each material type. In particular, it is well known that in an energy trapping piezoelectric device, the resonant frequency of spurious vibrations, called an anharmonic overtone, is present near the resonant frequency of the principal vibration. The anharmonic overtone is not excited when the electrode size (the energy trapping range) is small. The maximum electrode size where the anharmonic overtone is not excited depends on type of material used. Accordingly, it is necessary to determine the maximum electrode size for each type of material.
Unoriented bismuth layer-structure compound type ceramic materials have an excellent thermal stability. Thus, it would be expected that such structures could be used to achieve high performance piezoelectric devices. However, for the energy trapping piezoelectric devices operable to excite a thickness extensional third harmonic vibration, the maximum electrode sizes where anharmonic overtones are not excited have not been determined.
Accordingly, it is an object of the present invention to provide a piezoelectric device with which a high performance oscillator with an excellent thermal stability can be realized.
In accordance with the first aspect of the invention, the piezoelectric device comprises a piezoelectric substrate and at least a pair of vibration electrodes formed on opposite faces of the piezoelectric substrate. The piezoelectric substrate comprises of piezoelectric material containing as major components Sr, Bi, Nb and O. The vibration electrodes are formed so as to oppose each other and define an energy trapping region wherein a thickness extensional third harmonic wave is excited when an appropriate signal is applied across the pair of vibration electrodes. The energy trapping region is defined between the area of overlap of the vibration electrodes as viewed along a plane which runs parallel to the plane of the vibration electrodes. The length L of the longest secant extending between two intersections on the periphery of the energy confining region and the distance t between the pair of vibration electrodes satisfy the relationship L/t less than 9.
Instead of using Sr, Bi, Nb and O as major components of the piezoelectric material, Sr, Bi, Ti and O or Ca, Bi, Ti and O can be used as the major components. Preferably, SrBi2Nb2O9 is employed as a major component.
A piezoelectric material containing Sr, Bi, Nb, and O as major components, such as SrBi2Nb2O9 is thermally stable. If such a material is used to produce a piezoelectric device operable to excit a thickness extensional third harmonic vibration, and the value L/t is set to be less than 9, superposition of an anharmonic overtone can be avoided, and good energy-trapping can be performed.
The value L represents the maximum length of a secant extending between two intersections on the periphery of the energy confining region. This distance is measured along a plane which runs parallel to the planes of the vibration electrodes.
If the plan shape of the energy trapping range is circular, the reference character L represents the diameter of the circle. If the shape is elliptic, L represents the longer axis of the ellipse. If the shape is a rectangle or square, L represents the length of the diagonal.